1. Field of the Invention
The invention relates generally to magnetic disk drives capable of constructing an indicated position signal from an integer track component and a fractional track component or fractional PES segment. The invention relates more particularly to a method of constructing an indicated position signal that is continuous from fractional PES segment to fractional PES segment and a related method of linearizing the fractional PES segments without destroying the continuity.
2. Description of the Related Art
A conventional disk drive has a head disk assembly (xe2x80x9cHDAxe2x80x9d) including at least one disk (xe2x80x9cdiskxe2x80x9d), a spindle motor for rapidly rotating the disk, and a head stack assembly (xe2x80x9cHSAxe2x80x9d) that includes a head gimbal assembly (HGA) with a transducer head for reading and writing data. The HSA forms part of a servo control system that positions the transducer head over a particular track on the disk to read or write information from that track.
The industry presently prefers a xe2x80x9crotaryxe2x80x9d or xe2x80x9cswing-typexe2x80x9d actuator assembly which conventionally comprises an actuator body that rotates on a pivot assembly between limited positions, a coil that extends from one side of the actuator body to interact with a pair of permanent magnets to form a voice coil motor, and an actuator arm that extends from the opposite side of the actuator body to support the HGA.
Each surface of each disk conventionally contains a plurality of concentric data tracks angularly divided into a plurality of data sectors. In addition, special servo track information is provided on this disk or another disk to determine the position of the head. A manufacturing fixture called a servo track writer (STW) is used to write the servo track information on the surfaces of the disks in an HDA. The STW mechanically moves the actuator to a given reference position precisely measured by a laser interferometer. The HDA is then driven to write servo track information for that position. The process of precisely measured displacement and servo track writing is repeated to write all required servo tracks across the disk.
The most prevalent servo system used in disk drives is called xe2x80x9csampled servoxe2x80x9d or xe2x80x9cembedded servoxe2x80x9d wherein the servo track information is written in a plurality of servo sectors that are equally angularly spaced from one another and interspersed between data segments around the track. Each servo sector comprises a track identification (ID) field defining a gray code track number or servo track number and a pattern of servo burst fields. The transducer head reads the track ID field and the servo bursts to construct an indicated position signal formed from an integer component and a fractional component. The gray code track number (track ID) provides the integer component and the servo bursts provide the fractional component. The difference between the indicated position and a desired position forms a position error signal (overall PES) for use by the servo control system. Note that overall PES is different than the fractional component which is often simply called the xe2x80x9cPES.xe2x80x9d This application will refer to the latter as a xe2x80x9cfractional component,xe2x80x9d xe2x80x9cfractional PES,xe2x80x9d or xe2x80x9cfractional PES segmentxe2x80x9d to avoid confusion.
The servo control system reads the track ID field and samples the servo bursts to position the transducer head at the desired radial position. The servo control system moves the transducer head toward a desired servo track during a coarse xe2x80x9cseekxe2x80x9d mode using the track ID field as a control input. Once the transducer head is generally over the desired servo track, the servo control system uses the servo bursts to keep the transducer head at a desired radial position relative to the servo track in a fine xe2x80x9ctrack followxe2x80x9d mode.
One conventional servo burst pattern is a repeating pattern of four bursts, grouped as two burst pairs, where each pair is abutted along the xe2x80x9cburst pair centerlinexe2x80x9d and wherein the pairs and associated burst pair centerlines are offset from one another by a fixed amount. The servo system constructs a fractional PES by computing a pair difference signal for each pair and choosing which pair difference signal to use based on which pair is closer.
The pair difference signal is zero if the head is positioned at a xe2x80x9cseamxe2x80x9d or burst pair centerline where a pair of bursts are abutted. The pair difference signal increases as the head moves away from the burst pair centerlines. In one conventional embodiment which makes efficient use of the STW, each burst is nominally one data track wide and the burst pair centerlines are spaced apart by one half of a data track. Because the read width of the head is less than a burst width, the servo system must switch or xe2x80x9ccommutatexe2x80x9d between burst pairs when the head is at a xe2x80x9ccommutation positionxe2x80x9d near the mid-point between burst pair centerlines. Commonly, a fractional PES from a burst pair at track center is termed xe2x80x9cPxe2x80x9d or primary, while a fractional PES from the adjacent burst pair is termed xe2x80x9cQxe2x80x9d or quadrature. In another embodiment used with narrower read heads such as magnetoresistive (MR) heads, the burst width may be two/thirds of a data track width and the burst pair centerlines are spaced apart by one/third of a data track width.
The indicated position signal should be continuous through the commutation position. An indicated position signal of ordinary construction, however, may have discontinuities owing to variations in the placement of the bursts during the servo writing process, non-linearity of the read head signals relative to true displacement from a burst pair centerline, or both. Discontinuities are especially prevalent with the current use of magnetoresistive (MR) heads, and are especially troublesome. First, MR heads have an inherently non-linear microtrack profile. Second, MR heads suffer from xe2x80x9chead switch instabilityxe2x80x9d in that when a head is selected, it may have undergone a state change which causes the head to exhibit a gain which is significantly shifted from its nominal value. Finally, to make matters worse, the servo system often operates with the head near the commutation position and in the vicinity of a potential discontinuity when xe2x80x9cmicro-joggingxe2x80x9d (operating the head at an offset from a burst pair centerline) in one case to compensate for the offset between the read and write elements in an MR head.
The inventors are aware of MR head disk drives having servo control systems which provide some degree of hysteresis at the commutation position. Such disk drives tend to continue deriving the fractional PES from one burst pair centerline, or the other, even if the head moves a short distance to the other side of the commutation position. The hysteresis, therefore, reduces the likelihood of encountering a discontinuity while operating at the commutation position. Disk drives using hysteresis, however, continue to construct the fractional PES in a conventional manner such that a significant variance may still exist in the indicated position signal on either side of the commutation position. Accordingly, while hysteresis helps resolve the effect of discontinuities in the indicated position at the commutation position, it does nothing to prevent such variance in the first place and continues to allow for inherent instability while operating the MR head at or near the commutation position.
U.S. Pat. No. 5,825,579, assigned to IBM, is an example of others having tried to achieve a continuous PES at the commutation position. The IBM inventors, however, select one of four segments +P, +Q, xe2x88x92P, or xe2x88x92Q based on whether the head is over xe2x80x9ctrack typexe2x80x9d 0, 1, 2, or 3, and then xe2x80x9cstitchxe2x80x9d the segments together to form a position error signal (PES) portion of the overall position signal (fractional PES segment herein). Moreover, the IBM approach requires different algorithms for wide read heads (equation 5), narrow read heads (equations 6 and 7), and read heads with widths between these two extremes (function 11). The IBM approach is apparently head width dependent because it relies on linearizing the raw signal components rather than the overall position signal and its fractional PES segments.
There is a need therefore for an improved method of constructing an indicated position signal which is continuous through the commutation positions, is insensitive to head width and to discontinuities caused by servo burst variations which occur during the STW process, and impervious to gain variations caused by changes in MR head channel characteristics.
Finally, disk drives generally require some method of linearity compensation. This is particularly true of disk drives using MR heads. If such compensation is applied to the overall position signal, however, it must not destroy the continuity previously provided for at the commutation position. There is a further need, therefore, for a xe2x80x9cconstrainedxe2x80x9d method of linearity compensation which provides the desired compensation without modifying, or by equally modifying, the predetermined transition values determined from the burst pair centerlines on either side of the commutation position.
In one aspect, the present invention may be regarded as a method of determining an indicated position signal corresponding to a radial position of a read head over a rotating disk in a magnetic disk drive, the method implemented in a disk drive including a rotating disk and a read head for reading servo information from the rotating disk, the servo information including a servo pattern of four circumferentially successive, radially repeating servo bursts A,B,C,D that are arranged in quadrature to define an alternating plurality of AB and CD burst pair centerlines that are nominally spaced from one another by xc2xd servo track pitch units, the method comprising the steps of: reading four servo bursts A,B,C,D with the read head; determining whether the read head is closer to an AB burst pair centerline or to a CD burst pair centerline; and causing the fractional track PES value to equal xc2x1xc2xc track pitch units at a commutation position midway between the AB and CD burst pair centerlines regardless of whether or not those centerlines are separated by xc2xd servo track pitch units by constructing a fractional PES value as   P  =            -              1        4              ⁢                  (                  C          -          D                )                    (                  A          -          B                )            
when the read head is closer to the CD burst pair centerline within a first fractional PES segment and as   P  =            +              1        4              ⁢                  (                  A          -          B                )                    (                  C          -          D                )            
when the read head is closer to the AB burst pair centerline within a second fractional PES segment the first and fraction PES segments continuously connected to one another at the commutation position to form an indicated position signal; and linearizing the fractional PES values within the fractional PES segments while keeping the fractional track PES values equal to one another at the commutation positions and keeping the indicated position signal continuous.
In a second aspect, the present invention may be regarded as a disk drive having a disk, a transducer, and an embedded servo control system including a servo sector having a first burst pair and a second burst pair disposed on the disk, a method of constructing a position signal reflecting a position of the transducer relative to the servo sector, the method comprising the steps of: obtaining a measured position p relative to a first burst pair centerline wherein the measured position p ranges within a first fractional PES segment from a first measured endpoint position pSTART on a first side of the first burst pair centerline to a second measured endpoint position pEND on a second side of the first burst pair centerline, the first and second measured endpoint positions being equal in magnitude and opposite in sign such that the first fractional PES segment is continuous at a segment junction formed with a second fractional PES segment associated with a second burst pair centerline located adjacent to the first burst pair centerline; and deriving an estimated position {circumflex over (x)} that more closely corresponds to a true position x by compensating for nonlinearities in the measured position p at positions with values in between the first and second measured endpoint positions, while keeping the first and second endpoint values equal in magnitude and opposite in sign to keep the first fractional PES segment continuous with the second fractional PES segment.
In a third aspect, the invention may be regarded as a disk drive having a disk, a transducer, and an embedded servo control system including a servo sector having a primary burst pair joined at a primary burst pair centerline and a quadrature burst pair joined at a quadrature burst pair centerline, wherein the servo control system derives a primary position signal waveform from the primary burst pair and a quadrature position signal waveform from the quadrature burst pair, wherein the servo control system provides fractional PES segments that are subject to discontinuity when they are joined at a commutation position to form an indicated position signal, a method for producing a discontinuity-corrected position signal from the fractional PES segments, the method comprising the steps of: determining whether the transducer is closer to the primary burst pair centerline or the quadrature burst pair centerline; and if the transducer is closer to the primary burst pair centerline, constructing a fractional PES value from a first ratio of the primary and quadrature position signal waveforms; else if the transducer is closer to the quadrature burst pair centerline, constructing the position signal from a second ratio of primary and quadrature position waveforms; and applying a linearity correction to the fractional PES value which is applied independently of a width of the transducer and regardless of whether the transducer is nearer either burst pair centerline or the commutation position; whereby the indicated position signal is continuous at the commutation position and substantially linear between the commutation position and an adjacent commutation position.